What causes a P0491 secondary air injection code on Chrysler
Trouble code P0491 Secondary Air Injection Excess/Insufficient Air Flow is related to a problem with the secondary air injection system. To understand the purpose of the secondary air system and to understand how it works, read this post
The P0491- Secondary Air Injection–Excess/Insufficient Air Flow trouble code can be cause by a bad pump, bad check valve or a restriction in the tubing between the pump and the exhaust manifold, or a restriction in the manifold itself. The system uses the Oxygen sensor to detect a lean condition during cold startup. The computer calculates the proper air/fuel mixture based on engine temp and MAF sensor readings. By pump extra air into the exhaust pipe ahead of the oxygen sensor, the system creates a lean condition and the PCM expects to see that. Depending on the make, model, and engine, the PCM can test the system by quickly turning the pump on and off to see what readings come back from the O2 sensor.
On some Chrysler engines, the feed pipe from the pump directs air into chamber cast into the cylinder head and there is a port drilled from it into each individual exhaust port,. Any restriction in flow to this chamber will cause a low flow code.
If the port is clogged, the O2 sensor won’t see the additional air. And, if the cylinder head has ever been replaced with an incorrect head, it may not have the drilled port.
If the pump turns on and air is moving through the one-way valve, but the code still appears, look for a restriction or missing ports.
©, 2015 Rick Muscoplat
Posted on by Rick Muscoplat